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This is where you start. I was lucky enough to have a stock TPI base available to make a template from for the distributor hole. It doesn't have to be a TPI base as all Gen1 SBC's use the same distributor hole. 





Here is the template I made from a piece of aluminum. It picks up the last intake bolt on each side. I then used a pencil to mark the hole and drilled it to 1 3/8".






A clearer picture.





Picture of the LT1 intake with the distributor hole drilled.

This is what I used to drill the 1 3/8" hole. It's called a counterbore tool, start with a 1/4" hole and then go through with the counterbore. I was able to borrow it from work as it is rather expensive to buy for a one-time project. 




Next you must address the distributor height issue. The Gen1 SBC intake is 1/4" taller then the LT1 intake. 





See the gap? This is the LT1 intake. You will have to make a spacer of some sort. I had my neighbor turn a 1/4" thick aluminum washer 1 3/4" diameter with a 1 3/8" hole on his lathe. Unfortunately I don't have a pic but you get the idea.